This invention relates to the processing of seismic data and is particularly directed to the compensation of seismic data for the frequency dependent attenuation by the earth of propagated acoustic signals.
In utilizing the Vibroseis (a trademark of Continental Oil Company) seismic process, a seismic source signal is generated comprising an oscillatory, multi-frequency signal having predetermined starting and ending frequencies which are nonrepetitive over a preselected time period, and which is substantially longer than the travel time of such a signal as it is reflected from a subsurface geological strata to a seismic receiver on the earth's surface, such a signal to be hereinafter referred to as a "sweep" signal generated by a "vibrator source." Such a process is disclosed in U.S. Pat. Nos. 2,688,124 (Doty et al) and 2,989,726 (Crawford et al), and utilizes an oscillatory "sweep" signal as disclosed therein and which basic disclosure is incorporated by reference herein. As further used herein, the sweep signal generated by the vibrator source and coupled for transmission into the earth will be referred to as the "transmitted sweep" signal, and the sweep signal recorded as a counterpart to the transmitted sweep signal will be referred to as the "recorded sweep" signal.
The frequency dependent absorption of compressional waves in the earth is a known phenomenon quantitatively exhibited on seismic data obtained when making a Vibroseis survey. One method of correcting such data for the high frequency attenuation effects exhibited is disclosed in U.S. Pat. No. 2,808,577 (Crawford et al). The method disclosed therein is directed to a process for altering the effect of at least a portion of either the source "sweep" signal or the recorded propagated data signal upon the correlation values of such signals to counteract the effects on the correlation values which are due to the unequal attenuation by the propagating medium (the earth) of the high frequency components of the transmitted source signal. However, there is no disclosure or even suggestion of a technique for determination of accurate quantitative values of attenuation over the seismic source signal bandwidth nor how to quantitatively compensate for such attenuation.
Prior laboratory studies indicate frequency dependent absorption that is approximately linear at frequencies of 100 Hz and above. Hamilton, E. L., Compressional Wave Attenuation in Maritime Sediments, GEOPHYSICS, Vol. 37, pp. 620-646 (1972). Extrapolation to seismic bandwidth is uncertain but could be reasonably expected. Attewell, P. B., and Ramana, Y. V., Wave Attenuation and Internal Friction as Functions of Frequency in Rocks, GEOPHYSICS, Vol. 31, pp. 1049-1056 (1966). However, such studies have only measured or predicted the attenuation of compressional waves in certain sedimentary rocks, and such studies have not suggested any technique for accurately determining the attenuation function of received seismic data obtained as a result of using a Vibroseis sweep signal over the sweep signal bandwidth, nor any technique to quantitatively weight a Vibroseis sweep signal prior to correlation to compensate for such attenuation by the propagating medium.
Accordingly, one primary feature of the present invention is to provide a process for accurately determining the frequency dependent attenuation function exhibited by seismic data over the entire frequency range of the vibrator source signal.
Another feature of the present invention is to provide a process for accurately and quantitatively determining the compensation function to be applied to the source sweep signal prior to correlation with the received propagated data signals in order to compensate for the attenuation by the propagating medium.
Yet another feature of the present invention is to determine the discrete times for applying the derived compensation function to the source sweep correlation signal.